The present invention relates to electronically commutated electric machines, to which phase voltages are applied in order to drive the electric machine. Furthermore, the present invention relates to methods for sensorless detection of an armature position of the electric machine.
Polyphase electronically commutated electric machines, in particular synchronous motors, can be driven, for example, by providing a stator magnetic field which leads an excitation magnetic field produced by an armature. The stator magnetic field, in particular the direction and strength thereof, is produced by applying phase voltages to winding phases of the stator of the electric machine.
In order to be able to determine the lead of the stator magnetic field, knowledge of the armature position of the armature of the electric machine is required. Often, in the case of such electric machines, the armature position is determined without the use of sensors in order to save on a position sensor. One possible method consists in measuring the induced back-emf in the winding phases and determining the time of the zero crossing of said back-emf. The time of the zero crossing of the induced back-emf can be used for the determination of the armature position (back-emf method).
In order to measure the induced back-emf, it is necessary to ensure that no external voltage is applied to the respective winding phase. During driving of the electric machine with the aid of block commutation, however, a driving phase voltage is generally applied to the corresponding winding phase at any point in time, with the result that this sensorless method cannot be used or further measures are necessary for measuring the induced back-emf despite block commutation.
Therefore, when using block commutation, a driver circuit which generates phase voltages for driving the electric machine is generally driven in such a way that, in a time window in which the induced back-emf is intended to be measured, no external voltage is applied to the respective winding phase or the corresponding phase connection for the winding phase is switched so as to be freely floating, with the result that the induced back-emf can be measured. The time window in which a voltage potential or a phase voltage is not applied to the respective winding phase is referred to as a blanking interval and is selected such that the zero crossing of the induced back-emf is included. The time of the zero crossing of the induced back-emf can be uniquely associated with a determined armature position and used to determine the times of the switchover between drive patterns for application of the phase voltages to the winding phases of the electric machine.
In the case of continuous-operation machines such as pumps or fans, it is often sufficient, owing to limited demands being placed on dynamics, to detect only the zero crossing of the induced back-emf of a winding phase. There is therefore overall an inexpensive possibility for sensorless operation of such pumps and fans.
The converted block commutation and the provision of the blanking interval result in a high level of noise development, however, which arises as a result of the abrupt commutation changes/switchovers. The commutation changes result in changes in phase current with a relatively steep edge. The resultant torque ripple and radial force excitations result in clearly perceivable noise.
When using such electric machines in electric vehicles, the consideration of noise emission is important because the noise of the apparatuses which are equipped with such electric machines is more markedly perceivable in this case since this noise is no longer drowned out by a relatively loud internal combustion engine.